drm/loongson: Run DRM default client setup

Call drm_client_setup() to run the kernel's default client setup
for DRM. Set fbdev_probe in struct drm_driver, so that the client
setup can start the common fbdev client.

The loongson driver specifies a preferred color mode of 32. As this
is the default if no format has been given, leave it out entirely.

v5:
- select DRM_CLIENT_SELECTION

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sui Jingfeng <suijingfeng@loongson.cn>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-68-tzimmermann@suse.de